In the color image forming apparatus of the electro-photographic method using an intermediate transfer body, an apparatus in which a toner image formed on an image carrier which is a photo conductor is transferred on the intermediate transfer body, and the toner image on the intermediate transfer body is transferred onto a transfer material (called also recording sheet, or sheet), is well known. In such a color image forming apparatus, after the toner image successively formed on the image carrier and charged into a predetermined polarity is transferred on the intermediate transfer body by being superimposed by a primary image transfer unit by using an electrostatic force, the toner image on the intermediate transfer body is collectively transferred onto the transfer material by using an electrostatic force by a secondary image transfer unit.
Because a charging amount per one particle of a toner is almost uniform, the electrical potential on the intermediate transfer body is determined by toner adhesion amounts in a predetermined area, and in the color image forming apparatus, the charge potential of a part the toner image on the intermediate transfer body where a plurality of colors of toners are superimposed, is higher than a charge potential of a part where only one color toner is adhered.
When a fluctuation of the potential on the intermediate transfer body after the primary image transfer is large, various image failures are easily generated. In Patent Documents 1, 2, it is proposed that, uniform secondary image transfer can be conducted, by uniforming the charge amount of the toner on the intermediate transfer body by charging the toner image on the intermediate transfer body before the secondary image transfer by a corona discharge of AC or DC.
[Patent Document 1] Unexamined Japanese Patent Application Publication No. 10-274892
[Patent Document 2] Unexamined Japanese Patent Application Publication No. 11-143255
In such a proposition, the charged amount of the toner on the intermediate transfer body is uniformed to a large amount. However, when the toner layer potential is high, the density fluctuation is easily generated by the transfer charge shortage, and when the transfer charge is made large, the disturbance of the image by discharging is easily generated. Accordingly, the present inventors are studying on uniforming the charge amount by discharging the toner image on the intermediate transfer body before the secondary image transfer.
Toner layers on the intermediate transfer body have various cases from 1-layer to multi-layers, and in order to not too-much discharge the toner and to charge the appropriate charge, the present inventors are studying on the uniform charging by the scorotron discharge.
The scorotron discharge is conducted in such a manner that: a grid to which voltage is applied is arranged with a gap of about 1 mm against the intermediate transfer body(belt), and from the back of the body, the discharge by a discharge electrode using wire is conducted. Further, on the back surface of the intermediate transfer belt on which grids are arranged, the opposite electrodes are arranged.
Eelectrically grounded rollers which wind the intermediate transfer belt are excellent as the opposite electrode in a point that the rollers reduce a load of the intermediate transfer belt, however, when the linear speed of the intermediate transfer belt is large in the high speed machine, it is necessary that the grid width opposite to the intermediate transfer belt need to be large in order to obtain a sufficient discharge effect. However, when a place having a curvature such as a roller is made an opposite electrode, there is a limit in increasing the width of the electrode under the condition that a gap between the intermediate transfer belt and the grid is kept within a certain range, and there is a problem that the toner charge can not be controlled.
Accordingly, a method that the electrically grounded electrode is arranged being opposed to the plane part of the rotating intermediate transfer belt, is applied.
When, an opposite electrode is arranged opposite to the part of the plane of the rotating intermediate transfer belt, the opposite electrode is fixed and the opposite electrode slides on the belt surface from the back surface of the intermediate transfer body. For the opposite electrode, an electric conductive brush or an electric conductive sponge is used, and for the purpose to make the conductivity through the opposite electrode, in order to make the contact state good, it is necessary that the belt back surface is pressed by the opposite electrode with a certain degree of loading. The intermediate transfer belt is rotated under the condition of being tensioned, and the width of the intermediate transfer belt need to be 300 mm or more to accommodate to sheets of A4 size. In that case, the difference of the stress is generated to the moving direction between the central part and the end parts, as the result, when pressed by the opposite electrode, the central part of the belt is pushed, and the phenomenon that the belt is raised to the grid side, is generated. The gap between the intermediate transfer belt surface and the grid varies depending on the position of the belt width direction, and the discharge efficiency varies between the central part of the belt and the end part, and the discharge control for uniforming the charge on the toner image cannot be conducted.